Human convalescent antiserum to respiratory syncytial virus (RSV) administered intraperitoneally to cotton rats prior to RSV challenge provided near-complete protection from pulmonary infection. Antiserum given subsequent to viral challenge at the peak of virus replication, reduced pulmonary viral titer 10-2 or greater within 24 hours. Sandoglobulin, a preparation of purified human IgG with high titer of RSV neutralizing antibodies, produced the same effects as convalescent antiserum. Sandoglobulin was absorbed rapidly and produced a significant therapeutic reduction in virus titer within 3 hours. The degree of reduction of virus in pulmonary and nasal tissues was directly proportional to the titer of neutralizing antibodies attained in the serum of the recipient cotton rat and the extent of reduction was always greater in the lungs than the nose. Studies in the owl monkey showed that the intravenous administration of RSV antibodies at the time of maximum viral shedding reduced pulmonary viral titer an average of 10-1.5 within 48 hours, and cleared detectable virus from 75% of the animals. In spite of relatively high doses of antibody (3 gm/Kg) the treatment was well tolerated and did not exacerbate pulmonary disease. On the basis of these two studies, clinical trials of antibody therapy in infants hospitalized with RSV pneumonia are currently in progress at Children's Hospital National Medical Center. Therapy of RSV lower respiratory infection can also be achieved by administration of RSV antibodies directly into the lungs. The amount of antibodies required for a therapeutic effect is one three hundredth that needed when RSV antiserum is administered parenterally.